Improved nitrogen sparing was demonstrated in 20 patients undergoing either head-and-neck operations or abdominal explorations, who were randomized to receive intravenously either 3 per cent amino acids or 5 per cent dextrose. Infusions were started immediately after operation and continued for a minimum of six days. In patients receiving amino acids, as compared with those receiving dextrose, mean cumulative six-day nitrogen losses were significantly lower (42 plus or minus 5 g [S.E.M.] and 74 plus or minus 7 g, respectively--P smaller 0.005), as were serum glucose and insulin levels, but beta-hydroxybutyrate, acetoacetate and blood urea nitrogen were significantly elevated. No adverse effects of the amino acid solution were observed. The presumed mechanism for improved nitrogen sparing is a decrease in serum glucose and insulin levels, allowing greater endogenous fat mobilization and utilization, thus sparing lean body mass.